But make sure he flies coach.

[John Doe]

but make sure he flies coach.

What's That Mean? 

Thx. 

[Thomas]

Coach and first class are the two type of seats in an airplane, first class being the most expensive choice!

I don't know if it is "technically" grammatically correct or not, but it's very common to see both coach and first class as adverbs, like your example sentence. You also see them as adjectives, such as "How much is a coach seat?" -- and as nouns, "How much is a seat in coach?" ("coach" in this case meaning "the coach section")

[John Doe]

Thx a lot.

You're the very useful boss here!